when i've done HPI before one of the main issues is that you are only covered if you have completed it before you buy it and have completed all sections inc mileage. i think there was a week overlap from buying where you had to go back on and add the mileage to make sure you are fully covered.
if you haven't done the HPI or do it after buying it your not covered at all. your only way out is to hope the garage help.
persoanlly i think your less likely to see a full refund as most cars are sold as seen, if there HPI is clear; at time of sale then it wass down to the buyer to do their own.
a meet half way on price situation may be more likely outcome.
but to be honest you'll need to play it by ear. if the gargae are happy to refund you the full amount, then go that route, if they odn't seem that helpful then the paying the differnece route is better,